The present invention relates to an induction heating device for inductively heating an object to be heated which is formed of conductive material.
The invention also relates to an induction heating fixing device of induction heating type for fixing to a sheet a toner image formed on the sheet while conveying the sheet.
The invention also relates to an image forming apparatus having an image forming unit for forming a toner image on a sheet and an induction heating fixing device of induction heating type for fixing to the sheet the toner image formed on the sheet while conveying the sheet having the toner image formed thereon by the image forming unit. Among image forming apparatus of this type are copying machines, laser printers, facsimiles and the like, typically.
Recently, fixing devices of induction heating type that achieve high thermal conversion efficiencies have been proposed in terms of energy saving.
For example, a fixing device disclosed in patent literature (Japanese Patent Laid-Open Publication 2002-93566) has a heating roller (a member to be heated including a metal sleeve) that is rotated by a motor, a pressurizing roller that is in pressure contact with the heating roller, and a coil that is provided along part of outer periphery of the heating roller and that is tightly wound so as to form a layer. A thermistor (temperature detecting means) is provided so as to face a part of the outer periphery of the heating roller wherein the part is far from a region which the coil faces. In operation, a high-frequency current is fed through the coil and the heating roller is heated by an induced current (eddy current) caused thereby. The temperature of the heating roller is controlled so as to be held at a predetermined temperature on basis of detection signal from the thermistor. A sheet is conveyed while being nipped between the heating roller and the pressurizing roller and a toner image formed on the sheet is fixed to the sheet.
The fixing device, however, has a defect in that it is difficult to accurately detect the temperature because the part of the outer periphery of the heating roller of which temperature is detected by the thermistor is far from the region (heating region) which is inductively heated by the coil. In particular, failure in the motor might cause the heating roller to stop rotating and to undergo an abnormal local temperature increase, which could not be detected accurately in the above example and might entail a danger of firing.
In such an external coil type in which a coil is placed outside a heating roller as described above, it is often impossible to provide a temperature sensor inside the heating roller because thermal insulating material such as sponge rubber is provided inside the roller. In an internal coil type in which a coil is placed inside a heating roller, it is possible to provide a temperature sensor outside the heating roller so as to face a heating region, however, the placement of major elements inside and outside the heating roller results in increase in scale and cost of the device.